Construction of the 4-Azafluorenone Core in a Single Operation and Synthesis of Onychine

This study describes the synthesis of the 4-azafluorenone core in a single operation using readily available starting materials. Condensation of an amidrazone with ninhydrin intercepts an intermediate 1,2,4-triazine derivative, which engages norbornadiene in a merged [4 + 2]/bis-retro[4 + 2] sequence to deliver the azafluorenone core. The tricyclic core established in this manner was elaborated to onychine, the simplest natural product in the 4-azafluorenone alkaloid family.

4-Azafluorenone natural products have been isolated from plants in the soursop (Annonacae) family 1,2 and other plant species. 3Representative alkaloids that possess this tricyclic fused system include onychine (1), polyfothine (2), and cyathocaline (3), and these and related metabolites possess biological activities including antimalarial and cytotoxicity (Figure 1). 4 Extensive synthetic exploration of azafluorenone derivatives has revealed enhanced antiproliferative characteristics or other properties as compared to the natural products. 5s examples, synthetic derivatives 4 and 5 offer improved anticancer activity as compared to the natural azafluorenone variants and are lead compounds for development and potential clinical application. 6,7Interest in the 4-azafluorenone core has not been restricted to medicinal chemists; the fundamental photophysical properties have also attracted significant attention. 8,9−13 Given the broad interest in 4-azafluorenones, several methods for the construction of the nitrogen-containing tricyclic fused system have been reported.If we narrow our focus to onychine (1), the simplest and earliest isolated member of the azafluorenone family, we may view the natural product as an instructive model that has been a proving ground of distinct synthetic approaches to the core structure.Synthetic efforts directed toward 1 and related natural products have been recently reviewed. 4ur interest in azafluorenes and azafluorenones, as well as in the de novo synthesis of pyridine structures, primed us toward a retrosynthesis that identified the substructure of ninhydrin (8)  embedded within the target molecule (Scheme 1: Retrosynthesis). 14,15Given the long and rich history of ninhydrin 16 derivatives, and the commodity status of the parent ninhydrin (8), we envisioned that a synthesis proceeding from this starting material would be direct and relatively versatile for incorporation of substituents on the pyridine nucleus.In this way, condensation of 8 with an amidrazone resembling 9 could lead to an intermediate 1,2,4-triazine 7, which would enable inverse-electron-demand Diels−Alder (IEDDA) and tandem cycloreversion and rearomatization to deliver the azafluorenone core. 17We anticipated that 2π-alkyne equivalent reaction components such as enamines, represented by structures 10a and 10b, would not only enable rapid construction of diverse 4-azafluorenone derivatives, but the complementary electronic features of 10a and 10b would allow us to modulate the regioselection in the cycloaddition event and permit isolation of 6 with favorable isomeric purity.Although 6-keto-1,2,4-triazines have been prepared in similar fashion, to the best of our knowledge, these precursors have not been explored in tandem [4 + 2]/retro [4 + 2]  sequences. 18,19Thus, we also saw an opportunity to enter new chemical space quickly and probe the reactivity of a tricyclic triazine resembling 7.
In order to evaluate the synthetic viability of the proposed sequence, we selected ethyl oxalamidrazone (11) with which to begin our studies (Scheme 2).We chose this amidrazone for several reasons: (1) 11 is well-known, reasonably stable, and benefits from a detailed Organic Synthesis procedure for preparation. 20(2) We reasoned that the electron-withdrawing ester function would lower the reaction threshold for the IEDDA on the derived triazine 12. Lastly (3), once the azafluorenone was prepared, the ester could serve as a handle to introduce other functionalities 21−24 or be resected 25 to prepare 2-protio azafluorenone structures such as 1−3.
Synthesis of triazine 12 proceeded efficiently (87% yield) through condensation of amidrazone 11 and ninhydrin (8) (Scheme 2).Given the routine use of enamines in IEDDA reactions with 1,2,4-triazines, our preliminary exploration likewise employed enamines as 2π reaction components. 17nfortunately, our results with either in situ prepared or preformed enamines, enamides, or enol ethers 26 derived from acetone or propanal were unimpressive.This exploration of electron-rich alkyne-equivalent heterosubstituted dienophiles failed to offer acceptable yields or regioselectivity.Our best result, using enamine 13 generated in situ according to the conditions reported by Taylor and co-workers 27,28 gave a 19% yield of an inseparable mixture of isomeric azafluorenones (1:1 isomer ratio).The starting triazine 12 was not returned from these reactions and the mass balance was heterogeneous in constitution and other products from the mixture could not be clearly identified.We suspect that the 6-keto functionality in triazine 12 frustrates the desired cycloaddition chemistry with enamine-type dienophiles, leading to poor cycloaddition reaction efficiency.
Given the disappointing results with enamines, we were pleased to discover that a non-nucleophilic alkyne-equivalent 2π reaction component, norbornadiene (14), was competent in reaction with triazine 12 and gave azafluorenone 15 in 87% yield on heating in ethanol overnight (80 °C, 16 h).Because both the condensation leading to triazine 12 and the pericyclic cascade to azafluorenone 15 (cycloaddition, and sequential extrusion of N 2 and cyclopentadiene) are performed in ethanol, we were able to telescope these reactions into a single operation.In practice, we found it most expedient to prepare azafluorenone 15 in this manner, using a single reaction vessel as illustrated in Scheme 3.
The use of norbornadiene ( 14) was enabling and provided facile access to the 4-azafluorenone core 15.With this structure in hand, we turned our attention to functionalization of the pyridine nucleus in 15.A variety of strategies were envisioned (e.g., pyridinium activation, 29−31 directed metalation, 32,33 Minisci reaction 34 ) and some strategies are partly supported by precedent on similar structures.We found that the hindered 2,6-bis-substituted pyridine nucleus in 15 precluded some chemistry, but the ester at C2 could serve as an effective blocking group that permitted regioselective functionalization at C4 using classic methodology enabled by pyridine N-oxide chemistry.To this end, the derived N-oxide of azafluorenone 15 was prepared (providing intermediate S1) and decomposed with POCl 3 to give the 4-chloroazafluorenone 16 (2 steps, 60% yield).Nucleophilic substitution of malonate (Cs 2 CO 3 , DMF, 80 °C, 85% yield) on 16 (giving intermediate S2) preceded acid-promoted hydrolysis of all ester functional groups and concomitant decarboxylation gave the 4-methyl-2-carboxy azafluorenone 17 (91% yield).Excision of the carboxylic acid functionality at the pyridine C2 position in 17 was performed under silver catalysis (15 mol % Ag 2 CO 3 , DMSO, 180 °C). 25 Under these reaction conditions, which are modified slightly from the literature, the azafluorenone natural product onychine (1) was cleanly obtained.Spectroscopic data for our synthetic material 1 is in agreement with data from other syntheses. 4n conclusion, we have discovered a direct method to prepare the core 4-azafluorenone structure in a single reaction vessel from readily available starting materials.The condensation operations with amidrazone and ninhydrin intercepted an intermediate 1,2,4-triazine and also proved compatible with the pericyclic reaction steps.Use of norbornadiene as the alkyne-equivalent 2π reaction component with the intermediate 1,2,4-triazine was integral to a successful cycloaddition.The ensuing cycloreversion steps (extrusion of N 2 and cyclopentadiene) proceeded in tandem fashion without isolation of intermediate adducts.The 4-azafluorenone product obtained in this fashion was elaborated to onychine, a model natural product in this family.The "one pot" construction of the azafluorenone core disclosed in this note is efficient and complementary to existing synthetic strategies.We are optimiztic that the efficient construction of the 4-azafluorenone core central to this work will enable downstream advances in both the medicinal and material applications of this privileged scaffold.
■ EXPERIMENTAL SECTION General Experimental Considerations.All reactions were carried out under an atmosphere of nitrogen in flame-dried or oven-dried glassware with magnetic stirring unless otherwise indicated.Dichloromethane was distilled from CaH 2 prior to use.All other reagents were used as received unless otherwise noted.Flash column chromatography was performed using P60 silica gel (230− 400 mesh).Analytical thin layer chromatography was performed on SiliCycle 60 Å glass plates.Visualization was accomplished with UV light, anisaldehyde, ceric ammonium molybdate, potassium permanganate, or ninhydrin, followed by heating.Film infrared spectra were recorded using a Digilab FTS 7000 FTIR spectrophotometer. 1 H NMR spectra were recorded on a Varian Mercury 400 (400 MHz) spectrometer and are reported in ppm using solvent as an internal standard (CHCl 3 at 7.26 ppm) or tetramethylsilane (0.00 ppm).Proton-decoupled 13 C NMR spectra were recorded on a 100 MHz spectrometer and are reported in ppm using solvent as an internal standard (CHCl 3 at 77.0 ppm or DMSO at 39.5 ppm).All compounds were judged to be homogeneous (>95% purity) by 1 H and 13 C NMR spectroscopy unless otherwise noted.Mass spectra data analysis was obtained through positive electrospray ionization (w/ NaCl) on a Bruker 12 T APEX−Qe FTICR-MS with an Apollo II ion source using an ICR (ion cyclotron resonance) ion trap mass analyzer.

"One Pot" Method for the Preparation of Azafluorenone 15.
A dry flask was charged with ethyl oxalamidrazonate (11) (1.93 g, 14.6 mmol) and ninhydrin (8) (2.61 g, 14.6 mmol) and dissolved in EtOH (40 mL).The flask was fitted with a reflux condenser, flushed with N 2 , and heated to reflux using an aluminum block.After heating at reflux for 1 h, to the reaction was added norbornadiene (14) (6.7 mL, 73.0 mmol).Reaction heating was continued for an additional 16 h, after which time the reaction was cooled to 23 °C and concentrated in vacuo.The resulting mixture was purified by flash column chromatography as indicated above to afford 15 (2.23 g, 8.8 mmol, 60% yield) as a yellow powder.Spectral data for 15 is described above.

The Journal of Organic Chemistry Ethyl 4-Chloro-5-oxo-5H-indeno[1,2-b]pyridine-2-carboxylate (16).
A flask was charged with 2-(ethoxycarbonyl)-5-oxo-5Hindeno[1,2-b]pyridine 1-oxide (2.93 g, 11.0 mmol) and fitted with a condenser.To the reaction was added POCl 3 (30 mL) and the reaction vessel was heated using an aluminum block set to 100 °C.After heating for 3 h, the reaction was cooled to 23 °C and poured onto a mixture of ice (ca. 100 g) and CH 2 Cl 2 (50 mL).The pH of the aqueous layer was adjusted to 8 by adding solid K 2 CO 3 .The organic layer was removed and the aqueous layer was extracted with CH 2 Cl 2 (5 × 10 mL).The combined organic layers were dried with Na 2 SO 4 , filtered, and concentrated in vacuo.The resulting mixture was purified by flash column chromatography on silica gel (gradient elution: 0% EtOAc to 40% EtOAc in hexanes) to afford chloropyridine 16 Onychine (1).The following silver-catalyzed protodecarboxylation is based on related decarboxylation of pyridine-2-carboxylic acids. 25o a vial was added azafluorenone 17 (0.050 g, 0.21 mmol), Ag 2 CO 3 (0.008 g, 0.032 mmol), and DMSO (0.420 mL).The vial was sealed with a Teflon cap and heated using an aluminum block set to 170 °C.After 48 h, the reaction vessel was cooled to rt, diluted with sat.aqueous NaHCO 3 (3 mL) and extracted with CH 2 Cl 2 (5 × 3 mL).The organic layers were combined, washed with brine (2 × 2 mL), dried with Na 2 SO 4 , filtered and concentrated in vacuo.The resulting mixture was purified by flash column chromatography on silica gel (gradient elution: 0% EtOAc to 20% EtOAc in hexanes) to afford 1 (0.030 g, 0.137 mmol, 65% yield) as a light yellow solid.The spectral data obtained for 1 ( 1 H and 13 C NMR tabulated below) are in agreement with that reported in literature. 4